This invention relates generally to the field of gas lasers, and more particularly, to pumping techniques utilized to obtain a population inversion in gas lasers.
Gas lasers and their characteristics have been well known in the art for sometime, beginning with the helium/neon laser. In general, the population inversion required in order to obtain lasing in the gas is established in one of several ways two of which are: (1) By electron impact, i.e., directly transferring energy to gas atoms or molecules via electron collisions; (2) By means of a resonant energy transfer between a colliding donor atom and an acceptor atom or molecule to be lased.
The object of such energy pumping is to produce large densities of atoms in a particular excited energy state, which may subsequently radiate to one of a large number of lower energy levels. The optical transition of the atoms from the excited energy state down to any particular lower energy level is characterized by a transition probability or gain. Therefore, if a population inversion is obtained in the gas, then lasing will normally occur on the transitions having the largest probabilities. Indeed, this is the situation with most gas, liquid (dye) and solid state lasers. Unfortunately, this also limits the lasing wavelengths obtainable from a particular atom (or molecule). For a wide variety of atomic and molecular gas lasers, this restraint can be removed using the invention described here. This invention will allow these lasers to oscillate at more wavelengths, hence making them more versatile.
By way of example and not by way of limitation, these restraints on the wavelength attainable from a gas laser and the present technique for removing these restraints will be illustrated in terms of the argon/nitrogen laser. However, the use of the argon/nitrogen laser in the discussions to follow should not be construed in any manner as limiting the invention to that particular laser or even to molecular gas lasers.
Prior to discussing the lack of control of the spectral lines obtained with the Ar/N.sub.2 laser, it may be helpful to review the general mechanics of the Ar to N.sub.2 energy transfer used to obtain an N.sub.2 population inversion. It is possible, of course, to directly populate the upper electronic energy levels in N.sub.2 via the transfer of energy by collisions with high energy electrons. However, such a pumping technique is highly inefficient because it excites electrons in the N.sub.2 molecules to a variety of vibrational levels in a variety of energy states depending on the amount of energy transferred in the individual collisions. Accordingly, a large number of the vibrational levels in the various electronic energy states will be populated at any given time, thereby preventing effective control of the spectral lines emitted during electron transitions down to lower energy states.
However, as is well known in the art, if the gas to be lased is mixed with a doner gas having a metastable energy state approximately coincident with the electron energy state selected for population in the lasing gas, then a highly efficient indirect energy transfer may be obtained between the gases to populate this selected energy level in the lasing gas. (A metastable energy state is defined as a state wherein the electrons have a relatively long mean lifetime before they fall to the ground state).
A high concentration of donor gas (by an order of magnitude or more) relative to the acceptor gas or gas to be lased is utilized to selectively populate via resonant energy transfer the coincident energy level of the gas to be lased. More specifically, when the donor/acceptor gas mixture is pumped, a large density of the donor gas in its metastable state is produced. As each excited donor atom or molecule collides with an acceptor atom or molecule, a transfer of all of the excited energy from the donor to the acceptor takes place via a resonant or excitation energy transfer. Because of the high concentration of donor gas in the gas mixture, a significant population of acceptor atoms or molecules is excited up to the energy level coincident with the donor metastable state. Such an indirect pumping technique is both controlled and highly efficient.
In the Ar/N.sub.2 laser, this indirect pumping via the excitation of the argon gas is utilized to good effect. FIG. 1 is a partial energy level diagram of the argon atom juxtaposed with the partial energy level diagram for the nitrogen molecule N.sub.2. Only the lower vibrational levels in selected electron energy states are shown for Ar and N.sub.2 for ease of explanation. The .sup.3 P.sub.2 and the .sup.3 P.sub.o energy states are metastable in argon while the .sup.3 P.sub.1 and .sup.1 P.sub.o energy states are effectively metastable, i.e., the lifetimes of the electrons in those P states are long compared to the time required for excitation transfer of energy to N.sub.2. It can be seen that these P metastable states of argon are approximately coincident with the several vibrational levels of the C.sup.3.sub..pi..sbsb.u electronically excited energy state of the nitrogen molecule N.sub.2. By pumping the Ar/N.sub.2 gas mixture with a relativistic electron beam or beams, a significant portion of the argon atoms will be excited to the metastable P states. These excited agon atoms will, in turn, collide and transfer their excitation energy to a significant portion of the N.sub.2 population. Thus, a significant population of N.sub.2 molecules will be excited to their v'=0 vibrational level of the C state, thus creating a population inversion between the C state and the lower lying B level.
A significant amount of experimentation with the Ar/N.sub.2 laser has been reported since 1974 using this technique. A discussion of these laser experiments may be found in the following articles; Nelson, Mullaney, Byron, Applied Physics Letter 22, 79 (1973); Searles and Hart, Applied Physics Letter 25, 79 (1974); Basov, Vanilychev, Dolgikh, Kerimov, Dobanov and Suchkov, JETP Letters 20, 53 (1974); Ault, Bhaumik and Olson, IEEE, J. Quant. Electron, QE-10, 624 (1974). The standard pumping procedure in the experiments has been to excite a high pressure (greater than one atmosphere) Ar/N.sub.2 gas mixture with electron beams of 100 ns or less in length. For example, Searles and Hart utilized a pulse at 500 keV with a length of 50 ns. Likewise, the Ar/N.sub.2 laser described in U.S. Pat. No. 3,970,914 to Webster utilized a pulse at 1 MeV with a length of 20 ns. In these various lasing experiments with the Ar/N.sub.2 laser, emissions were observed on the 0.fwdarw.1 transition of the C.fwdarw.B band energy level at 357.7 nm. In at least one experiment, the work by Searles and Hart, weak emission was also observed on the 0.fwdarw.2 transition at 380.5 nm. (See also Ernst, Tittel, Wilson and Marowsky, J. Applied Physics 50, 3879 (1979).) The apparent restriction of the Ar/N.sub.2 laser to the 357.7 nm wavelength with some weak emissions at the 387.5 nm wavelength severely limits the versatility of this particular laser. Previous attempts to produce stimulated emissions from the 0.fwdarw.3 transition of the N.sub.2 C.fwdarw.B band at the 405.9 nm wavelength using transverse excitation of the Ar/N.sub.2 mixtures by an approximate 50 ns FWHM e-beam have been unsuccessful (see Eden, Chang, Palumbo, IEEE J. Quant. Electrons QE-15, 1146 (1979)), although one group of researchers did observe weak lasing on the 0.fwdarw.3 transition in a beam stabilized discharge. (See Nelson, Mullaney and Byron, Applied Physics Letters 22, 79 (1973).)